OP, I am going to go against the popular census on this thread and say YANBU. OK, a little BU as in two wrongs don't make a right, and flashing won't make a bit of difference to these ignorant and selfish drivers, but you are right - you are sticking to the rules of the road and I despair at the lack of thought of these other road users (sadly very common) - I will admit I have beeped occasionally at a completely unnecessary tailgate then overtake as it's dangerous and I've reacted to let them know that. I do the speed limit. I don't drive under it (also a bug bear!), but that's not good enough for all the self important folk out there who would rather intimidate and drive dangerously rather than leave a few minutes earlier (and the amount of times this happens means they all can't possibly be attending an emergency). Do I live in India or Mexico, famed for perilous roads? No, a leafy outer London suburb! Clearly there are a couple of self important drivers on this thread:
"Slow drivers sitting permanently on or just below the speed limit on single lane roads can be as much of a hazard as speeders. YABU."
Agreed, going slow is annoying, but perhaps they are not as familiar with the road or the driving conditions which leads to more cautious driving on occasion? Rules are rules and you cannot have a go at someone who drives the limit itself! But I'm sure this poster knows the roads around them. I tell people who want to drive faster to fill their boots and go to Germany and get on the autobahn.
I think we all have a friend like "Scatty"who don't like having to drive at the limit as they know they possess Stig like driving qualities, and if only those pesky limits and the people that insisted on sticking to them were eliminated, roads would become a highway of delight. No, because even if a 30 was raised to 40, they would do 50 (they are already doing the 40 as they know that's right!) I have been called Driving Miss Daisy by a friend whose entire family have had speeding tickets. I don't rely on my car for work - I am lucky enough to be able to drive at weekends, but if I couldn't access a car, it wouldn't be that bad - the people I know who have this superior attitude are often people (IME) who would never use public transport and generally rely on their vehicles to get to their place of work. Why risk it?
I've had friends like the PP who speeds on the motorway when 'conditions are right' and will not be told they are in the wrong. Slightly outing, so will be vague, but a friends sibling had a terrible crash on the motorway (flipped over several times) and is trying to sue the other party for the crash (they were miraculously unscathed) - I don't know who was to blame, I was not there, and it may have happened either way, BUT he had his cruise control set to 80 (early morning, knew the road, conditions etc) - he is in the wrong. My friend will not have it, laughable.
Sorry, have ranted! But driving is a privilege and not a right and I don't like being mocked for sticking to the rules (I hate driving at 20 MPH for the record where this has been done, but feel sorry for the PP who feels bullied by others for trying to adhere - that's not on).
So don't flash OP, but know that you are morally victorious!